In a process of forming a semiconductor device, a pattern such as a line and space (groove) pattern, a hole (opening) pattern or the like which is used for forming, e.g., a gate electrode, a wiring or the like, on a substrate, e.g., a semiconductor wafer (hereinafter, simply referred to as “wafer”) is formed by etching a plurality of films laminated as etching target films on the wafer by converting a predetermined processing gas to a plasma while using a patterned photoresist film as a mask.
At this time, for example, when a SiON (silicon oxynitride) film as an etching target film is etched, a processing gas including a gas containing a CHF-based gas such as CHF3 or the like is used (see, e.g., Japanese Patent Application Publication No. 2003-163349).
The SiON film has good optical characteristics in a far infrared ray region. Therefore, when a pattern is formed in a photoresist film by a photolithography technique, the SiON film effectively serves as an antireflection film for preventing reflection from an underlying film.
However, if such a SiON film is plasma-etched by the processing gas including the gas containing a CHF-based gas while using the photoresist film as a mask, the surface of the photoresist film becomes rough. The surface roughness of the pattern sidewall is transferred when the SiON film is etched, which causes a so-called striation in which the pattern sidewall of the SiON film (sidewall of line) becomes irregular (concave-convex). If the striation occurs, the leakage current between wirings may be increased, or defects such as short circuit or the like may occur. Therefore, there is a need to prevent the striation.